At the present time, there are a variety of devices on the market for temporarily supporting cars and trucks in an elevated position. Many are used for repairing or maintaining one's own car. Of these, the most common are hydraulic jacks, jack stands, and drive-on ramps. Jacks lift a car to provide working space underneath and jack stands support a jacked-up car. Drive-on ramps both lift and support a car. Perhaps the most attractive are the drive-on ramps because of their appearance of stability. In most cases, these devices are made from steel or, in some cases wood, because the need for strength is paramount.
Prior art ramps have been found to be unstable and prone to move away from a vehicle as it is being driven on. This movement is caused by the relatively small amount of surface contact made with the ground and the narrow base of current prior art structures. Metal ramps have been found to damage asphalt or other surfaces because they gouge into the surface and do not evenly displace the forces applied to them. Some currently available metal ramps actually instruct the user to avoid asphalt because it is too soft for the sharp metal edges. Alternatively, the metal side walls may be subjected to lateral forces as the ramp slides or sticks on the surface, causing collapse of the ramp since metal has an excellent ability to withstand compressive forces but does not resist bending well at all.
One solution to the problem has been proposed in our U.S. Pat. No. 5,033,146, issued Jul. 23, 1991. In this patent, a light weight, high strength vehicle service ramp is described in which a system of interlocking vertical cross members running longitudinally and transversely throughout the length and width of the structure provides support from side-to-side and front-to-back. The preferred structure is manufactured from dense wood such as marine grade plywood. Alternatively, it is suggested that a high strength plastic type compound could be used to form the vehicle service ramp.
As shown in our above-identified U.S. Pat. No. 5,033,146, a honeycomb like arrangement is made by setting various pieces together and attaching them to the side and back walls to form the completed ramp. Initial products of this design were manufactured from wood, as noted above, and they were found to be quite suitable as these prototype ramps were used daily in several facilities. Attempts to transform the design from wood to plastic resulted in a number of difficulties. Efforts included the use of extremely expensive engineering grade plastic resins. Attempts to create a method for molding the product led to changes in design and totally impractical production methods and materials. This endeavor was therefore totally unsuccessful in producing a vehicle service ramp from high strength plastic type compounds as suggested in our above-identified patent.
Other difficulties were encountered in contemplating the manufacture and sale of plastic ramps. Pressed metal is extremely cheap and easy to fabricate, and thus it appears that plastic ramps would be more expensive then metal ones. Moreover, plastic has a particular problem unique to plastic in that it is known to creep, thereby causing a deformity of the polymer structure as weight is continuously applied. This defect does not occur significantly in wood or metal products.
Accordingly, an object of this invention is to produce a vehicle service ramp which does not have the inherent problem of creep, at least at weights which are expected to be encountered.
Another object of this invention is to produce a vehicle service ramp which is made from inexpensive materials.
Other objects will appear hereinafter.